Liquefied natural gas (“LNG”) generally refers to colorless, transparent cryogenic liquid converted from natural gas (predominantly methane) that is cooled to approximately −163° C. and condensed to about 1/600th of the original volume.
As LNG emerges as an energy source, efficient transportation means have been sought in order to transport LNG from a supply site to a demand site in a large scale so as to utilize LNG as energy. Resulted in a part of this effort are LNG carriers, which can transport a large quantity of LNG by sea.
LNG carriers need to be furnished with a cargo that can keep and store cryogenically liquefied LNG, but such carriers require intricate and difficult conditions.
That is, since LNG has vapor pressure that is higher than atmospheric pressure and boiling point of approximately −163° C., the cargo that stores LNG needs to be constructed with materials that can withstand a very low temperature, for example, aluminum steel, stainless steel and 33% nickel steel, and designed in a unique insulation structure that can withstand thermal stress and thermal contraction and can be protected from heat leakage, in order to keep and store LNG safely.
Described below with reference to the accompanying drawings is the insulation structure of a conventional LNG carrier cargo.
FIG. 1 is a sectional view illustrating a conventional insulation structure of an LNG carrier cargo. As illustrated, a bottom insulation panel 10 is adhered and fixed by way of a fixing plate 10a to an internal face of a hull 1 of an LNG carrier by epoxy mastic 13 and a stud bolt 14.
Here, interposed and adhered in between the bottom insulation panel 10 and a top insulation panel 20 is a rigid triplex 22. When the bottom insulation panel 10 is adhered to a cargo wall, the bottom insulation panel 10 is formed with a gap 40 so that a flat joint 18 made of a glass wool material can be inserted in the gap 40 formed between bottom insulation panels 10.
Then, a top bridge panel 28 is attached in between the top insulation panels 20 by adhering a supple triplex 26 over the rigid triplex 22, which is already attached, with epoxy glue 24 and then adhering the top bridge panel 28 over the supple triplex 26 with epoxy glue 24.
The top insulation panel 20 and an upper part of the top bridge panel 28 have a same planar surface, on which a corrugated membrane 30 is attached by way of an anchor strip 32 to complete the cargo wall.
Looking at how the internal face of the hull 1 and the bottom insulation panel 10 of an LNG carrier are assembled in further detail, the stud bolt 14 is adhered to an inner wall of the hull 1 by resistance welding, and a through-hole, through which the stud bolt 14 can be inserted, is pre-formed vertically in the bottom insulation panel 10.
Accordingly, assembly is completed by engaging a nut 14a with the stud bolt 14 and inserting a cylinder-shaped foam plug 15 in the hole formed in the bottom insulation panel 10.
As described above, in the conventional cargo insulation structure, the through-hole as shown in FIG. 1 or a through-cavity for performing the same function as the through-hole is vertically formed at a boundary of the insulation panel in order to fix the insulation panel to the hull by use of the stud bolt. In this case, discontinuity is formed in the insulation panel, inevitably lowering the airtightness of the sealing membrane and weakening the adhesive force to the supple triplex due to the reduction in the adhesive area of the upper surface.
Moreover, in case there is a crack in the epoxy glue for installing the supple triplex during the assembly of the insulation panel in the hull, the crack can be extended all the way to the through-hole or through-cavity for fastening the stud bolt, providing a path for gas leak that may adversely affect the hull.